I suggest if you have any uncertainties about fractions or percentages you have a look at those tutorials first. A ratio, like a fraction or a percentage is a way of describing the how many of a collection of things falls into one category. When using fractions we describe the proportion of something which falls into a certain category relative to the value 1 which is taken to represent the whole. In percentages we do a similar thing using 100 to represent the whole. When we use a ratio to describe a proportion we are relating the proportions to each other rather than to some arbitrary value which represents the whole.